On Tue, Jun 23, 2020 at 5:08 PM Guido van Rossum <gu...@python.org> wrote:
> [SNIP] > On Tue, Jun 23, 2020 at 12:11 PM Brett Cannon <br...@python.org> wrote: > >> I will say that trying to follow >> https://github.com/python/peps/blob/master/pep-0622.rst#runtime-specification >> was really hard. Any chance of getting some pseudo-code that shows how a >> match is performed? Otherwise all of that wording tries so hard to be a >> spec that I found it hard to follow in my head in how things function. >> > > Sorry about that. This section was subject to heavy editing recently and > lost clarity. I will try to make it better! Writing it as pseudo code will > take a little time, but I will give it a try. > I leave the decision about pseudocode up to you if you think you can just find words to clarify that section. > > >> For instance, "When __match_args__ is missing (as is the default) or >> None, a single positional sub-pattern is allowed to be passed to the call" >> is really misleading as it seems that a "sub-pattern" in this case is just >> going to be a constant like `[1, 2, 3]`. Otherwise how does `["<"|">"]` or >> `[1, 2, *_]` get represented as a "single positional sub-pattern" (if >> either of those examples is possible)? The use of the term "sub-pattern" >> feels misleading because while you may consider even constant patterns a >> "pattern", going that generic feels like any pattern should fit in that >> definition when in fact it seems to only be an object where a direct >> equality check is done. >> >> It seems the way things work is basically: >> >> 1. `__match__(obj)` returns a proxy object to have Python match against; >> it is passed in the thing that `match` is running against, returning `None` >> if it know there's no chance a match will work >> 2. If `__match_args__` is present, then it is used to map positional >> arguments in the pattern to attributes on the proxy object >> 3. From there the `match` functionality does a bunch of comparisons >> against attributes on the proxy object to see if the match works >> >> Is that right? That suggests all the work in implementing this for >> objects is coming up with a way to serialize an object to a proxy that >> makes pattern matching possible. >> > > Yes, that's right, and the protocol was defined carefully so that the > author of __match__ doesn't have to do any pattern matching -- all they > have to do is produce an object that has the right attributes, and the > interpreter does the rest. Note that it is __match__'s responsibility to > check isinstance()! This is because __match__ may not want to use > isinstance() but instead check for the presence of certain attributes -- > IOW, the class pattern supports duck typing! (This was a little easter egg. > :-) > Ah, so that's how you're going to have people simply match against protocols. :) > > >> One thing I see mentioned in examples but not in the `__match__` >> definitions is how mappings work. Are you using `__match_args__` to map >> keys to attributes? Or are you using `__getitem__` and that just isn't >> directly mentioned? Otherwise the section on how `__match__` is used only >> mentioned attributes and never talks about keys. >> > > Oh, __match__ is *only* used for class patterns. Mapping patterns are done > differently. They don't use __getitem__ exactly -- the PEP says > > Matched key-value pairs must already be present in the mapping, and not > created > on-the-fly by ``__missing__`` or ``__getitem__``. For example, > ``collections.defaultdict`` instances will only match patterns with keys > that > were already present when the ``match`` block was entered. > > You shouldn't try to depend on exactly what methods will be called -- you > should just faithfully implement the Mapping protocol. > > OK, so `__contains__` then. -Brett > -- > --Guido van Rossum (python.org/~guido) > *Pronouns: he/him **(why is my pronoun here?)* > <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/> > _______________________________________________ > Python-Dev mailing list -- python-dev@python.org > To unsubscribe send an email to python-dev-le...@python.org > https://mail.python.org/mailman3/lists/python-dev.python.org/ > Message archived at > https://mail.python.org/archives/list/python-dev@python.org/message/N7USX5OB2NEPO25JLTMTI4SUQ2CB7WLY/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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